For the material for shadow masks, cold-rolled sheet steel has heretofore been produced according to a process mentioned below. Specifically, low-carbon steel manufactured by steel manufacturers is subjected to finish hot-rolling at a finishing temperature not lower than the Ar3 transformation point thereof, then washed with acid and cold-rolled into a sheet having a predetermined thickness. Next, this is degreased, then subjected to decarburizing annealing in a wet atmosphere in a box-type annealing furnace, and optionally subjected to secondary cold-rolling to a reduction ratio of at least 50% so as to make it have a thickness of final products.
The cold-rolled sheet steel produced according to this process is photo-etched by etching workers, and then annealed for softening it and thereafter pressed to make it have a predetermined shape by pressing workers. Next, this is annealed in an oxidizing atmosphere for forming an oxide film, or that is, a so-called blackened film on its surface to thereby prevent it from rusting and to reduce its radiation ratio. One important characteristic that the sheet steel is desired to have is soft magnetism. Along with the inner shield therein, the shadow mask in TV Braun tubes acts to protect the linear motion of electron beams from the external magnetic field in the environment such as geomagnetism (this is hereinafter referred to as environmental magnetic field), and therefore it must be readily magnetized by itself in the environmental magnetic field. In addition, when the direction of TV is changed, the shadow mask is magnetized in the same direction in accordance with the environmental magnetic field, and therefore, it is desirable that the demagnetizability of the shadow mask is good. To satisfy the desired soft magnetic characteristics, it is desirable that the shadow mask material has a small value of coercive force (hereinafter this is simply referred to as Hc).
For reducing the coercive force of the shadow mask material, it is desirable to coarsen the crystal grains of the material. However, coarsening the crystal grains of the conventional shadow mask material is limited, and Hc of the material is from 103 to 135 A/m or so though depending on the annealing temperature thereof. The material does not satisfy the above-mentioned requirements.
Given that situation, an object of the present invention is to provide a shadow mask material which is superior to the conventional shadow mask material in point of the soft magnetism, especially having a remarkably lowered Hc to satisfy the ultra-soft magnetism necessary for shadow masks, and to provide a method for producing the material, a shadow mask and a picture tube.